1. Field of the Invention
The present invention relates to disc stacks used in drives, brakes and combinations thereof utilized in connection with the movement of tooling and other loads which are normally present in manufacturing environments, such as two-speed drives incorporating brake and clutch mechanisms which selectively interface with high and low speed electric motors. More particularly, the present invention relates to oil shear disc stacks having magnetically actuated clutching and/or braking functions in a drive, brake or combination thereof. Still more particularly, the present invention relates to oil shear disc stacks of the aforesaid class having improved performance due to fast response time as a result of minimization of residual magnetism when electrical current to the solenoid thereof is shut-off.
2. Description of the Prior Art
Movement of tools and other machinery in manufacturing environments is facilitated by drives which permit both rapid movement and slow movement, coupled with a brake function. Such drives permit rapid job cycling with accurate load positioning, while providing a maximum expected operational life of the drive. Typically, such drives interface with position locating apparatus which convert rotation into translation, such as by a ballscrew apparatus. Drives of this type incorporate two electric motors: a primary drive motor used for rapid movement and a secondary drive motor coupled through a gear reduction unit used for slow movement. These drives further incorporate a brake for precisely stopping movement and a clutch for selectively engaging the primary and secondary drive motors. The clutch is structured in the form of interleaved annular discs of two annular disc sets which collectively form a stack, in which one set of annular discs is attached to a shaft connected with the secondary drive motor, while the other set of annular discs is attached to a shaft connected with the primary drive motor, and a mechanism is used to selectively control clamping pressure between adjacent annular discs. The brake is structured analogously to the clutch, except that one annular disc set is attached to a stationary housing component.
The disc stacks utilized in the brake and clutch rely upon transmission of torque from one set of annular discs to the other set of annular discs. Some systems rely upon dry frictional engagement between adjacent annular discs to provide torque transfer, this is ordinarily considered unacceptable because of excessive wear and tendency for the dry friction material coating the annular discs to inconsistently rub relative to each other, resulting in a "stick-slip" jerking action which makes accurate positioning almost impossible to achieve. Systems which rely on dry frictional engagement between the annular disc sets generally are those which utilize low clamping force actuation systems, such as those which are electromagnetically operated. Accordingly, most conventional systems rely upon disc stacks utilizing an oil shear principle, in which adjacent annular discs brought into proximity by a clamping actuator, such as a pneumatic device, mutually transmit torque by a thin film of oil therebetween due to viscous shear of the oil film. The oil serves to reduce annular disc wear, provides consistent force transmission and conducts away waste heat. Accordingly, oil shear operated disc stacks have become industry standard, although there is involved a high degree of cost and installation complexity.
An example of a conventional two-speed drive is described in U.S. Pat. No. 4,463,841 to Kelley, dated Aug. 7, 1984. In this drive, a secondary electric motor drives through a gear reducer to an input shaft which connects with a clutch. The clutch is composed of an oil shear operated disc stack, in which one annular disc set is connected with the input shaft and the other annular disc set, interleaved with the first annular disc set, is connected with an output shaft. The output shaft connects with a primary electric motor, which, in turn, provides an output shaft for the drive. The output shaft interfaces with a brake composed of a second oil shear operated disc stack, in which one annular disc set is connected with the output shaft and the other annular disc set, interleaved with the first, is connected with the housing. A pneumatically operated hi-directional piston is axially moved to selectively apply annular disc clamping pressure to either the clutch disc stack or the brake disc stack. In the former mode, the secondary electric motor is able to drive the output shaft, while in the latter mode, rotation of the output shaft is braked. In the neutral position of the bi-directional piston, the brake is off, the secondary electric motor unconnected, and the primary electric motor is used as the prime mover. Selective movement of the bi-directional piston in concert with selective actuation of the electric motors enables an operator to achieve rapid and precise relocation of a load.
While the drive described in U.S. Pat. No. 4,463,841 operates acceptably in many situations, it has several significant problems which have been only partly addressed in the prior art.
One problem is that in the event of a power or fluid pressure failure, it is possible for the drive to continue spinning without the benefit of a brake. This problem was at least partly solved by a drive described in U.S. Pat. No. 4,607,736 to Kelley, dated Aug. 26, 1986, in which the brake stack is normally clamped by biasing action of springs on the bi-directional piston, and which biasing action is overcome as long as fluid pressure is maintained in the pneumatic lines. This solution pertains, accordingly, only to pressurized fluid operated brake systems.
Another problem is that an external supply of pressurized fluid must be provided to actuate the bi-directional piston. This problem was solved by a drive described in U.S. Pat. No. 4,739,865 to Yater et al, dated Apr. 26, 1988, which discloses a drive incorporating clutch and brake components as generally described above and further incorporating a self-contained hydraulic pump system for actuating the hi-directional piston that controls clamping of the disc stacks. Accordingly, this drive eliminates the need for an external pressurized fluid source. However, this drive has the added costs of an internally provided pressurized fluid source, and it is not suited for use in two speed drives as it requires a motor to be running at the input shaft in order to actuate the hi-directional piston.
It is clear that electromagnetic actuation systems for oil shear disc stacks have superior performance characteristics, less installation cost and less maintenance cost over air actuated systems. However, a disc stack operating via an electromagnetic actuation system is far more sensitive to the need for being built according to a preset tolerance in order for the armature of the electromagnetic actuation system to properly regulate clamping of the disc stack. The standard technique used in the art for providing what is an analogous equivalent of a disc stack having a predetermined cross-section within a preset tolerance is to assemble the annular discs of the disc stack, then utilize shims to offset the effect of annular disc thickness variations, the accumulation of which exceeds, above or below, the preset tolerance of the disc stack cross-section necessary for the magnetic actuation system to function as originally designed. This situation is vastly exacerbated when the oil shear disc stack requires field servicing due to failure or wear. A technician who replaces components of the stack is working on best guess estimates as to the disc stack cross-section after re-assembly. In all likelihood, the reconditioned disc stack will fail because it is operating outside the preset tolerance of cross-section for optimum operational performance and life.
A pre-packaged disc stack, particularly of the oil shear variety, having factory determined preset tolerance of cross-section which is easily installed as a unit at the factory into a drive, brake or a combination thereof, or, in the field, is easily removed as a unit from the drive, brake or combination thereof followed by a new pre-packaged disc stack of the class aforesaid being installed as a unit is described in the aforementioned U.S. Pat. No. 5,389,049.
A problem that has been observed in oil shear disc stack units of the class described hereinabove is that residual magnetism of components of the drive, brake or the like continues for a significant amount of time, typically on the order of three to five seconds, after electrical current to the solenoid has been switched off. This residual magnetism causes the armature component to remain closed with respect to the electromagnet until the residual magnetism has decayed sufficiently for the springs to effect release of the armature. Consequently, the disc stack will remain clamped for a significant duration of time after the electrical current to the solenoid has been switched off.
While residual magnetism is present in all electrically actuated devices, this problem is more noticeable with respect to oil shear disc stacks because of the high electrical current requirements needed to clamp the discs of the disc stack and the precise positioning that is respected. By way of comparison, dry friction disc stacks require less electrical current (as the friction between discs is much higher than that between discs of an oil shear disc stack), and accordingly the effects of residual magnetism are less noticeable.
Accordingly, what remains needed is provision for minimization of the effects of residual magnetism for oil shear disc stacks in drives, brakes, and combinational applications thereof.